Good sites with tab, newsletter, and free lessons;
http://www.guitarsite.com/
also for songs and tab;
http://www.roughstock.com/cowpie/songs/
As far as guitars....when I started playing seriously, I got a Martin because that's the sound my guitar heroes played (acoustically) and have never had to look back or apologize, and I'm still playing it 35 years later. It's been lots of miles, over 100 countries, and five continents. A good guitar is worth the extra money. It's in tune when it comes out of the case, while others with lesser are trying to get theirs tuned.......My point is if you want to sound like Clapton's unplugged, then get an OM body, if you want to sound like Willie, get a GC20, or Kris, a D18, or Cash, a D41, or Robert Earl, whatever he plays. No one can refute that a Strat sounds like a Strat, and a Les Paul sounds like a Les Paul.....the same is true in acoustics.
I agree at a $600 price point you are limited. Mr72 had a list with some options that I will concur with and would add Alvarez, particularly Yari's, if you can find them. Another commonly played by country bands is Takamini because of the built in electronics....here again, there are good ones and most aren't.
Good luck. I wouldn't compromise on the sound you want. You will learn much faster and play much better on a good guitar.
Cotton